The present invention relates to a control apparatus for controlling the torque of an induction motor, and particularly for an electric vehicle having field weakening control.
In general, it is known that the vector control method for computing and controlling magnetic flux phase can be used to control the torque of an induction motor in accordance with a torque command, such as disclosed, for example, in Japanese Patent Application Laid-Open Nos. 64-16283 and 63-171181.
The first prior art discloses a method of generating a current command for vector control. In particular, it controls the current flow to the induction motor in accordance with a current command, by feeding back the motor current before feeding out a magnetic flux current for producing a magnetic flux and the current command that is coordinateconverted to a torque current in proportion to the torque. The method allows the induction motor to produce the torque in accordance with the torque command.
The second prior art discloses a method of feeding out a voltage command for vector control. It controls the voltage to make current flow to the induction motor in accordance with the voltage command, by feeding back the motor current, coordinate-converting it to magnetic flux current and torque current, and computes current control in the same rotating coordinate system as the rotation of the magnetic flux before feeding out the voltage command. The method also compensates for mutual interference of the magnetic flux current and the torque current, the delay inherent in the current detector.
Both of these prior arts maintain linearity of the torque control of the induction motor, and cause the induction motor to produce a torque in accordance with the torque command, thereby keeping the torque following characteristic.
Prior art devices for controlling an induction motor use a field control that can decrease the magnetic flux with motor speed. Such field control prevents the voltage from becoming low if a back electromotive force of the induction motor becomes high as the motor speed increases. On the other hand, it is also known to decrease the magnetic flux when the induction motor generates low torque, which is hereinafter referred to as "torque field weakening control". Torque field weakening control has a merit that can increase the efficiency at low torque. It therefore is effective for the electric vehicle having the induction motor, in which the efficiency is highest priority.
However, it has been determined that phase deviation in the case of torque field weakening control adversely affects the output torque of the induction motor to a greater extent than ordinary field control. Thus, with torque field weakening-control, delays inherent in the speed sensor cannot be ignored, while with the ordinary field control they can.
It is important that the first and second prior arts described above be able to detect motor speed correctly, since both compute a reference magnetic flux phase in terms of the detected motor speed. If they use cheap speed sensors, however, delays sometimes occur in the detection of motor speed.
For example, one type of speed generator is affected by the insertion of an analog low-pass filter to eliminate radio frequency noises, which causes a delay in the detected motor speed. Such detection delay tends to occur at a transient time when the motor speed changes. Similarly, a type of encoder having few pulses has a digital detection delay at the transient time, since a pulse period and pulse width are coarse. Such delays in the sensors cause the reference magnetic flux phase to deviate from the actual value, so that the correct magnetic flux phase cannot be obtained.
As described above, prior art control devices do not take into account the detection delay of the motor speed at the time of transients, and therefore cannot maintain linearity of the torque control. As a result, the output torque of the induction motor deviates from the torque command in transient situations.
In particular, electric vehicles which use torque field weakening control of the induction motor for high efficiency suffer from significant problems of torque following controllability. Torque field weakening control adversely affects a large inertia vibration system between a body and the motor of the electric vehicle. The body vibrates minutely in acceleration running, thereby making it uncomfortable to ride in.